What Is the Doppler Effect?

The Doppler Effect is when sounds or waves change how they sound depending on whether something is moving toward you or away from you.

Imagine you're playing with your friend near a big, noisy toy train. When the train comes zooming toward you, its horn sounds much louder and higher-pitched, like it's shouting, “Hi! I’m coming!” But once the train passes by and starts going away, the same horn now sounds softer and lower, like it’s saying, “Bye! I’m leaving!”

This happens because the waves from the sound get squished together when the source is moving toward you, making them higher. When the source moves away, the waves stretch out, making them lower.

You might not realize it, but the Doppler Effect is all around you:

• A siren on a fire truck sounds higher as it comes closer and lower as it goes away.
• A bicycle bell seems to change pitch when someone is riding toward you or passing by.

It’s like when you’re on a swing, the faster you go, the more things around you seem to “change sound” in fun ways!